Many herding breed dogs (the most commonly affected in the UK are: All breeds of Collie, Shetland Sheepdog, GSD and, Australian Shepherd dogs - including cross breeds) have a genetic predisposition to adverse drug reactions involving over a dozen different drugs.

The most serious adverse drug reactions involve several antiparasitic agents, Ivermectin, Milbemycin (and related drugs), the antidiarrheal agent Loperamide (Imodium), and several anticancer drugs (Vincristine, and Doxorubicin, plus several others). These drug sensitivities result from a mutation in the multidrug resistance gene (MDR1 gene).

A test distinguishes between clear, carrier and affected dogs. Clear dogs have no copies of the mutant gene responsible for the condition and will neither develop the condition nor pass the gene on to their offspring. Carrier dogs have one copy of the normal gene and one copy of the mutant gene, they will not develop the condition, but will pass a mutant gene on to approximately half of their offspring. Affected dogs have two copies of the mutant gene that causes the condition, and therefore, will react adversely to certain drugs.Please refer to LABOKLIN for up to date information as new drugs are often added.